[pschlute]

Subject magnification is measured relative to the size of the sensor, so it does not stay the same

The magnification of any lens "at the sensor" is identical for a given FL irrespective of sensor size.

[Sykil]

Sure, magnification is the same on the intended image plane, but either way that has nothing to do with f-stop.

[swanlefitte]

The rest doesn't really have anything to do with my point, which is that if you have a 50mm wide angle lens with the same physical diaphragm as a 50mm normal lens, the 50mm wide angle will have a higher maximum f-stop (e.g. smaller maximum entrance pupil) due to the necessary differences in the way they're designed and the effect that has on the entrance pupil.

I am wondering if your terminology is used any place else. It is a paradigm shift I am not familiar with. I have never heard a 50mm designed to have an image circle that will cover a medium format as a "wide 50mm" before. I can see the terms utility in lens design, is it used elsewhere?

[Sykil]

Pentax 645 Wide-Angle Primes - Lens Reviews and Specifications - SLR and Interchangeable Lenses - Pentax Lens Reviews & Lens Database
Pentax 67 Wide-Angle Primes - Lens Reviews and Specifications - SLR and Interchangeable Lenses - Pentax Lens Reviews & Lens Database

[swanlefitte]

When I put the 50mm 645 and 6x7 lens on a k1 it magically becomes a normal and on a k3 a slight telephoto. The key feature is the large image circle which is a wide angle. Yet the links above relate to the standard definitions of wide being where the FL is smaller than the film/sensor size. (Is that just my interpertation?) A "normal" 50mm to you is a 50mm that vignettes on a medium format in this nomenclature appropriate to this paradigm. You use a different paradigm where saying a normal 50mm means AOV is limited. It is valid and not widely used. With interchangability of lenses between fullframe and apsc and the confusion it generates, it might be better to use it. A wide can cover narrow ( long or telephoto already confused) but a narrow vignettes on a wide.

[Sykil]

The lens doesn’t change; you’re just capturing a crop of it that represents a smaller field of view. The 50mm medium format lens is still a wide-angle lens.

Assuming a rectilinear lens, field of view of the image captured by the sensor can be determined by the sensor dimensions and focal length. From that you have rough categories of focal lengths that you can call wide, normal, narrow or whatever relative to that format. These are conventions that have developed over time mostly relative to the 35mm film format.

But wide, normal, telephoto, etc. are properties of the lens. A rectilinear 25mm designed for Micro 4/3 (e.g. image circle diameter just enough to cover the sensor diagonal) is not a wide angle lens. Ever. Not on a smaller sensor, where the image circle is cropped to a smaller field of view, and not on a larger sensor, where not only does it not cover the entire sensor — it is fundamentally not designed to capture light from that wide of an angle.

When the lens is specifically designed for that format, defining wide/normal/narrow by focal length relative to sensor dimension and by focal length relative to image circle diameter are basically the same thing. But, yes, that falls apart when you look at lenses in general across different formats and designs. In photography full-frame is often assumed, but formats are pretty diverse now in the enthusiast still photography world. More generally, those definitions refer to the lens because there are unique design considerations based on the angle you’re capturing, not only in terms of just capturing a rectilinear image of that angle, but also controlling aberrations and distortion, which is particularly tricky for a wide angle.